Poulsen Espersen posted an update 1 day, 14 hours ago
Lipophorin is also responsible for the redistribution of sterols from the fat body to other tissues during the larval wandering stage.The loading of sterols from the fat body to lipophorin, unlike in mammals, is LTP independent and follows a simple aqueous diffusion pathway. To date, two groups of genes have been found that appear to regulate this process.Magro is expressed and confined in the membrane of enterocytes and can hydrolyze cellular sterol and cholesterol esters.In mammals, sterols in their free form are reverse transported via ABC transporters; the same mechanism is believed to operate in insects. Similar to the preferential efflux of phytosterols over cholesterol in mammals, insects also selectively reverse transport some sterols more than others.Caterpillars preferentially reverse transport cholestanol over cholesterol, possibly through two ABC transporters, and, more interestingly, reverse transport was sensitive to the relative spatial arrangement of sterol atoms. The broader functions of these ABC transporters as they relate to sterol regulation should be investigated in more detail.All insects require a source for sterols, for multiple physiological purposes, and this requirement has been shown to affect foraging behavior in the context of several different environmental factors. However, in the other aphid species, cholesterol levels were low. This variation in cholesterol profile highlights different sterol metabolic abilities among insects that share a similar host plant and, in the case of the aphids, indicates that even closely related insects may have radically different sterol metabolic capabilities.Phloem sterol profiles have now been examined in four different plant species; in contrast to vegetative tissues, cholesterol tends to be the dominant sterol in the phloem. This might explain the high cholesterol level in one of the aphid reasch Apamin species and perhaps suggests that the other aphid species may be feeding on tissues other than phloem. An additional point about sterols in phloem is that they can exist in three formsfree, conjugated to fatty acids, or conjugated to sugarswith conjugated sterol making up at least twothirds of the total sterol pool. Glycosylated sterols would be soluble in the phloem, while free sterols and fatty acidconjugated sterols would likely be bulk transported using a carrier protein.How conjugation affects sterol use by insects has not been examined, but we suspect that conjugated sterols can be cleaved because most insect herbivores synthesize and release esterases. Insect symbionts can also impact insect sterol nutrition and use.Furthermore, given that cholesterol was not recovered in either species, these aphids likely do not use hydroxyecdysone as their molting hormone.Additional examples of fungal symbionts aiding in sterol nutrition have recently been shown, including for grape berry moths. However, we currently know very little about the role that bacterial flora might play with respect to sterol metabolism.This was a function of feeding exclusively on pea aphids, which have very low tissue sterol content.However, fitness was restored by feeding on plants or eating phytosterols or cholesterol.Thus, reproductive failure was clearly caused by a sterol deficiency in the male.With respect to male reproduction, sterols are critical for membrane remodeling when each syncytial spermatid is individually assembled into its own plasma membrane during spermatogenesis.